Several arrangements exist for accessing a plurality of telephone central office lines from a single station set. The oldest and most commonly used arrangement is an electromechanical key telephone where several central office lines are brought into a single station, connection between the central office lines and the telephone apparatus being made mechanically through a switch. In a typical key telephone, several central office lines are brought in parallel into a single telephone station, and the station is equipped with keys and switches in order to connect each of the central office lines, one line at a time, with a single telephone apparatus. Because several stations typically must have access to the same central office lines a key telephone system includes a distribution box which distributes the tip and ring wires from each central office line, in parallel, to the several telephone stations. When several stations share several central office lines, visual indicators are associated with the keys for signaling purposes to identify, for example, which central office line is ringing, which line is engaged, or which line is on hold.
An alternative arrangement is the electronic key system in which each telephone apparatus is accessed by only one pair of tip and ring wires. Switching between the various central office lines and the telephone stations is done in a central, electronically controlled switching matrix located in an electronic key service unit. Typically, the telephone station contains keys which enable the user to identify to the switching matrix which central office line the user wishes to have connected to that station. Signaling and visual status indicators are made available to stations via additional wires, which carry the necessary electronic signals to perform the central office line selection and control, and to turn on and off the visual indicators.
Although key-system services are popular among-a large class of customers, the provision of specialized wiring and distribution arrangements for key-system stations is expensive and inflexible. A recent approach to the provision of key-system services is to connect each station via an individual line to a stored program controlled, central office or private branch exchange (PBX) switching system and to have the central control of that system coordinate the processing of calls to and from members of key-system groups. Accordingly, no specialized wiring or distribution arrangements are required for the key-system stations.
One example of a central office switching system providing key-system services via individual lines is the integrated services digital network (ISDN) switching system disclosed in D. J. Ahnen et al. U.S. Pat. No. 4,791,662 on Dec. 13, 1988, where each key-system group comprises two or more customer stations that share a directory number even though they are connected via individual lines. Each group can include one or more ISDN lines and at most one conventional analog line. The capability to include one analog line in a key-system group is very important in applications such as work-at-home, where a customer has one analog line intended for personal voice communication and one ISDN line intended primarily for work-related voice and data communication but also usable for personal voice communication. This is an attractive arrangement because it is less expensive than multiple ISDN stations and lines. A typical application is shown in FIG. 26 where a central office switching system 110 provides service to a residence via an ISDN line 131 to an ISDN station 130 and an associated personal computer 140, and via a conventional analog line 121 to analog station 120. Note that ISDN station 130 has a directory number DN2000 (the "work number") with three call appearances CA1 through CA3, and a directory number DN1000 (the "home number") with a single call appearance CA1 that is shared with the single "call appearance" of analog station 120.
An important drawback of the prior art arrangement (FIG. 26) relates to the widely deployed call waiting feature. With conventional analog stations, a call waiting tone is transmitted to alert the station that a second call is incoming for the directory number. Flash-hook signaling is then used to connect the station to the second call and to subsequently connect the station to either the first or second call. With ISDN stations, the equivalent feature is implemented using multiple call appearances of the directory number, e.g., call appearances CA1 through CA3 of directory number DN2000. While a party is active on DN2000-CA1, a second call may be offered on DN2000-CA2 and the party can take the second call by depressing the button for DN2000-CA2. The first call is automatically placed on hold. The party subsequently returns to the first call by depressing the button for DN2000-CA1. With the prior art arrangement (FIG. 26) however, it is not possible to provide call waiting to analog station 120 having a given DN, e.g., DN1000, and to also offer multiple calls at multiple call appearances of the given DN at ISDN station 130.